Known techniques pertinent to a heat generating sheet making use of heat generation accompanying oxidation of oxidizable metal powder with air oxygen include the heat generating element of sheet form described in Japanese Patent 2572612. The heat generating element of sheet form is made by papermaking using a composition comprising iron powder, activated carbon, an electrolyte, water, and a fibrous material and features conformability to a part of the body and the like.
Such a heat generating element of sheet form is used by, for example, putting the heat generating element in an air-permeable holder and applying or attaching the holder to a part of the body. A heat generating molded article of this type loses flexibility because iron powder cakes with progress of the exothermic reaction. It becomes gradually hard and feels uncomfortable in long time use.
The applicant of the present application previously made a proposal on a thin type heat generating molded article in JP-A-2003-102761. One of the characteristics of the heat generating molded article is that it is extremely thin and yet exhibits excellent heat generation characteristics as a heat generating element. It has been hoped that the proposed heat generating molded article would be free of the above described reduction in wearing comfort, feel during use and etc.
The warming seat pad disclosed in JP-A-U-1-158762 is also included in warming devices making use of heat generation accompanying oxidation reaction of oxidizable metal powder with air oxygen include. The warming seat pad consists of a flexible and air permeable foamed sheet as a surface side material, a flexible and heat-insulating back side material, a heat generating composition which heats up on contact with air and is interposed between the surface side material and the back side material, and a covering material having ventilation holes which covers the surface side material.
The warming seat pad is designed to supply sufficient air to the heat generating composition by disposing the heat-insulating sheet formed of open-cellular resin foam between the covering material and the heat generating composition. However, the heat insulating sheet absorbs a water vapor generated by the oxidation reaction, which gives rise to the problem that the heat generating composition is not uniformly oxidized, resulting in reduction of heat generation duration or variation in temperature of generated heat.
The foamed sheet disposed as a cushioning material between the covering material and the heat generating composition not only interferes with conduction of heat from the heat generating composition to the surface but also makes the pad bulky and inconvenience to carry. Because the heat generating composition is powder, it can be localized while in use to cause appreciable non-uniformity in temperature of heat generation. To use a powdered composition limits the product size and therefore limits the usability of the product. Furthermore, the powdered composition makes a rasping sound inherent to powder or causes a discomfort when a user sits thereon. Therefore, the warming seat pad is unsatisfactory as a commodity.
The above-mentioned thin type heat generating sheet proposed by the applicant is extremely thin and yet exhibits excellent heat generating characteristics as a heat generating element as stated and also enjoys high productivity. The sheet is additionally characterized by reduced fall-off of constituent components from the sheet compared with a conventional heat generating element made up of powder.
A heat generating sheet of this type is usually combined with an air permeable sheet and an air impermeable sheet and used as a heat generating molded article. Unity between these sheets and the heat generating sheet is therefore one of important factors that decide the heat generation characteristics, the hand or texture, the usability of the resulting heat generating molded article, and etc.
The heat generating molded article of sheet form proposed by the applicant is broadly usable in view of its characteristics. In some applications, the sheet is required to reach high temperatures immediately after the start of use. In some other applications, two or more of the sheets are used in a stacked form. In such cases, it is demanded to prevent the stacked sheets from sliding off each other.
JP-A-11-56895 proposes forming a pattern in a bag holding a heat generating element by embossing, which offers no solutions to the aforementioned problems.
While the heat generating molded article of sheet form proposed by the applicant has broad applications for its characteristics, it is still desirable to have the preferred heat generation characteristics without increasing the thickness.